The manufacture of carbon black produces carbon black in the form of a fine dust-like fluffy powder. Carbon black in its powder form is difficult to handle and transport, as the product readily forms an airborne suspension of its finely-divided particles. Moreover, carbon black in powder form has a relatively low density and is not readily compactable, so that manufacturers or users of carbon black must handle and pay shipping charges on a relatively large volume of product to obtain a given mass of carbon black for various end-use applications.
It is known in the art to agglomerate the carbon black in powder form to form granules or beads of carbon black. The density of the resulting beaded carbon black product is greater than that of carbon black powder, permitting shipment of a greater mass per unit volume of beaded carbon black. Furthermore, carbon black in beaded form remains a fluent product for handling and shipping in bulk, while greatly reducing the amount of carbon black which becomes airborne during handling, and correspondingly reducing the health hazard and waste arising whenever carbon black in powder form is shipped in bulk.
Carbon black beading apparatus of the prior art typically includes a rotating drum mounted on a nearly-horizontal axis. A feed pipe extends some distance inside the rotating drum, with the feed pipe usually approximately coaxial with the drum. A feed screw is incorporated within the pipe, and carbon black in powder form is fed through the pipe and allowed to fall from the pipe outlet onto a bed of carbon black beads established within the rotating drum. Optionally, carbon black in beaded form is fed into the drum with the powder, if needed to maintain the desired relative proportion of beads within the drum. When the beader drum is initially started, carbon black in fluffy powder form is introduced to the drum, normally in conjunction with seed beads, until the inside wall of the drum is coated with the powder, and continuing rotation of the drum and flowing action of the powder therein starts forming a bed of carbon-black beads in the drum. Powder carbon black is added to form more beads, and the beads of carbon black are continuously withdrawn from one end of the drum.
The mixture of beads and powder in the drum forms a fluent bed within the drum, and this bed is carried partway up one side of the drum as the drum rotates. For example, if the drum when observed from the exit end is rotating counterclockwise, the bed of beads is carried up the right side of the drum to an angle determined by the type of material, the inner drum surface, and the diameter and rotating speed of the drum. The incoming carbon black powder falls from the outlet end of the feed pipe onto the upper surface of the bed existing in the drum, forming a top layer which actually flows to the left (assuming the foregoing example of counterclockwise rotation) due to the bed angle created by rotation of the drum. Because of this bed angle, the center of the bed in the drum is no longer directly beneath the outlet of the powder infeed pipe, and the incoming carbon black powder is thus deposited on what amounts to the leading edge of the bed within the drum. Because the top layer or surface of the bed flows in the direction toward the leading edge of the bed, the incoming carbon-black powder may be deposited on the drum surface prior to complete incorporation within the existing carbon-black bed of beads. The incoming powdery material thus tends to remain on the surface of the bed or to adhere to the drum wall, instead of becoming readily and completely distributed throughout the bed. More of the incoming dust thus becomes caked on the drum without attaching itself to the beads already formed within the drum. As the existing beads are being withdrawn from the drum, a condition called "dust out" can occur where substantially no beads remain in the drum. When this happens, the production of beads is suspended while drum rotation continues for the time required to reestablish a bed of beads within the drum. This bed rebuilding time is wasteful nonproductive down-time for the beader drum, thereby increasing the cost of producing carbon black beads.